This application claims the benefit of International Application No. PCT/GB00/01220 filed Mar. 30, 2000 and published in English, which in turn claims priority based on GB 9907725.7 filed Apr. 1, 1999.
This invention relates to signal processing apparatus of the kind in which an input signal is subject to both amplification and frequency conversion; and especially relates to radio telecommunications apparatus in which a voice signal is subject to amplification and frequency conversion.
The emerging GSM-EDGE and UMTS standards for mobile telecommunications place an increasingly stringent requirement on the linearity of handsets, particularly given their proposed wider channel bandwidths. In order to realise a power-efficient handset design, some form of linearisation will be required in the handset transmitter which should be (i) low-power itself; (ii) capable of broadband linearisation (up to 5 MHz for UMTS/ULTRA; (iii) frequency flexible, and preferably multi-band; and (iv) capable of achieving and maintaining high-levels of linearity improvement with highly-non-linear power amplifiers (e.g. class-C).
According to one aspect, the invention consists in a method of linearising an output signal comprising the steps of providing an input signal, digitally predistorting the input signal using polynomial distortion generation and frequency converting it in succession to provide a predistorted, frequency-converted signal, and amplifying the predistorted, frequency-converted signal to produce an output signal.
The trend in base-station technology is toward the adoption of xe2x80x9csoftware radioxe2x80x9d techniques, i.e. architectures in which all of the modulation parameters, ramping, framing etc. take place for all channels at baseband (digitally). The combination of all channels, at appropriate frequency offsets from each other, can also be performed at baseband and the whole spectrum up-converted in a single block for multi-carrier power amplification and the transmission from a single antenna.
However, the up-conversion and power amplification need to be linear (low-distortion) in order to prevent the radiation of unwanted adjacent channel energy and hence some form of linearisation is usually required for the power amplifier. In one embodiment of the present invention, the system incorporates a digital baseband (or digital IF) interface between a baseband signal generation sub-system and a linearised transmitter sub-system performing the above method.
With the invention it is possible to allow the transmitter to become a digital-in, RF-out system with the linearisation taking place in the form of digital predistortion.
In one embodiment, the predistortion of the input signal occurs prior to its frequency conversion. Advantageously, the frequency converting step is a frequency up-conversion step.
The input signal may be provided in quadrature form comprising in-phase and quadrature channels and the predistorting step may involve predistorting each channel independently.
Advantageously, the predistortion process may involve controlling the amplitude and/or phase of some part of the predistortion. This may involve controlling the predistortion to introduce a variation of amplitude and/or phase with frequency into at least a part of the predistortion.
In one embodiment, the predistortion may be controlled on the basis of a feedback signal derived from the output signal. In such an embodiment, it is possible to inject a pilot signal into the input signal and to monitor distortion of the pilot signal in the output signal as feedback. The feedback may be used together with the generated predistortion to generate control signals controlling the predistortion. The generation of these control signals may involve the step of correlating, or mixing, predistortion with feedback. It may be advantageous to perform the step of using the predistortion together with the feedback signal to generate control signals for the predistortion at least partly in the analogue signal domain.
The predistorting process may involve generating a distortion from the input signal and reintroducing the generated distortion into the input signal. The distortion signal may be generated by mixing or multiplying the input signal with itself. The step of generating a predistortion may involve generation of different orders of distortion by mixing the input signal with itself a different number of times. Advantageously, different orders of distortion can be controlled separately.
In a preferred embodiment, the frequency conversion and predistortion processes occur within a digital signal processor.
Any of the various methods described above may be used to generate an output signal for transmission from antenna means using an input signal which has been created in the digital domain and which contains information which it is desired to transmit.
According to another aspect, the invention relates to apparatus for linearising an output signal comprising predistorting means for digitally predistorting the input signal using polynomial distortion generation and frequency converting means operating in succession on an input signal to produce a predistorted, frequency-converted signal, the apparatus further comprising amplifying means for amplifying the predistorted, frequency-converted signal to produce an output signal.